Afghan Taliban fighters have seized the administrative headquarters of the Musa Qala district in Helmand Province after a week of heavy fighting, the latest in a series of setbacks for Afghan government security forces in the area.

The Taliban advance on August 26 came despite a series of NATO air strikes that killed about 40 Taliban fighters in the area, which is a major source of Afghanistan's opium crop.

Local officials who fled the town of Musa Qala overnight said hundreds of Taliban fighters merely regrouped after those air strikes and launched a renewed push late on August 25.

The capture of Musa Qala's district center by Taliban fighters also came after the militants overran nearby police and army posts as part of their summer offensive.

In recent weeks, the Taliban have also captured the neighboring districts of Nawzad to the west and Baghran to the north.

All three districts are in an area that saw some of the most lethal battles between Taliban militants and British troops in the NATO-led coalition force during more than a decade of fighting that followed the ouster of the Taliban regime in late 2001, after the U.S.-led invasion in the wake of the September 11 attacks.

Combat troops from Britain and other NATO countries pulled out of Helmand Province at the end of 2014, as the international force formally ended its combat mission in Afghanistan.

That left Afghan government forces to provide security for the territory that had been brought under their control at the cost of the lives of more than 400 British soldiers during the previous 13 years of fighting.

Musa Qala district chief Mohammad Sharif told RFE/RL on August 26 that the Afghan security forces were exhausted and stretched thin by trying to defend too much territory against Taliban assaults in Helmand Province in recent weeks.

He faulted the central government in Kabul for not providing fresh reinforcements or regular air support.

Sharif said he was forced to flee the district center overnight as Taliban fighters seized scores of government buildings in the area, including the district police headquarters and a health clinic.

The Musa Qala district includes fertile lands along the Musa Qala River, as well as a stretch of the Helmand River between the provincial capital, Lashkar Gah, and the strategic Kajaki dam.

It's an area where Afghan farmers produce a substantial portion of the country's lucrative opium crop, which is used to make heroin.

It has also been considered a Taliban stronghold for years, even while under government control, because of the support the Taliban receives from local villagers.

Razia Baloch, a female member of Helmand's provincial council, said recently that residents of the area had been supporting the Taliban because there have been no development projects in the area for years.

Independent analysts say thousands of civilian deaths caused by NATO air strikes during the alliance's 13-year presence in the area has also turned many residents against the Afghan government.

Abdul Hai Akhundzada, a member of Afghanistan's parliament from Helmand Province, told RFE/RL on August 26 that Afghanistan's security forces suffered heavy losses in the weeklong battle for Musa Qala.

He called on Afghanistan's government to take "serious measures" and launch an immediate counterattack to recapture the fallen district.